The present invention relates to a method of purification of phenol and in particular, to a method of purification of phenol which is produced within joint phenol and acetone production by the cumene method.
Phenol, after being distilled from heavy and light products of the joint synthesis, still contains the impurities of organic matters such as mesityl oxide, alpha-methylstyrene, 2-methylbenzofurane and other carbonyl compounds which are difficult to remove by conventional methods such as, for example, distillation. These impurities worsen the properties of commercial phenol since such phenol has low color stability.
A widely known commercial method of phenol purification from organic micro-impurities is a method based on phenol contact with ion-exchange resins as taught in Technology report: "Development of process scheme for phenol purification at ion-exchange KY-2, KY-2-8.gamma.c for Ufa Plant of Synthetic Alcohol", Giprokauchuk, Novokuibyshevsk, 1986, p. 84. The disadvantages of this method are a narrow temperature range of resin application (50.degree.-130.degree. C.), limited life of usage due to unrecoverable destruction of catalyst granules as well as problems of spent resin utilization.
U.S. Pat. No. 3,454,653 and U.S. Pat. No. 3,029,294 describe methods of phenol purification from impurities using aluminosilica catalysts. Disadvantages of these methods are low catalyst activity and, hence, long contact time (4-8 hours), as well as selective purification from one of the impurities components (in U.S. Pat. No. 3,454,653 from 2-methylbenzofurane, in U.S. Pat. No. 3,029,294 from hydroxyacetone).
The purpose of the present invention is the development of a catalyst with high activity for phenol purification from impurities with efficient regeneration, high mechanical strength and stability to give a long catalyst life.
This purpose is achieved by phenol purification using heterogeneous zeolite catalysts which are distinguished by the certain level of acidity, measured, for example, by a method of butane cracking as described in H. Rastlli, Can. Journal of Chem. Eng., 1982, v. 60, p. 44-49). All zeolites with K.sub.A &gt;10, where K.sub.A --first order reaction rate constant for butane cm.sup.3 /min*g can be used as catalysts for catalytic phenol purification in the liquid phase.
It is preferable to use zeolites which are designated according to classification of International Zeolite Association by indices FAU (zeolites X,Y,), MFI (for example, ZSM-5), MOR (mordenite), MAZ (omega), BEA (beta), FER (ferrierite) and others. These zeolites can be used with binders (aluminum oxide, silica gel, aluminosilicates or aluminophosphates) and without them. It is preferable to use zeolite of Y type with an aluminosilicate binder having a value of K.sub.A =50-80 cm.sup.3 /min*g and Si/Al ratio more than 3. Preferably, the concentration of sodium, potassium and other alkali agents on the basis of their oxides does not exceed 2 wt %, more preferably not more than 0.3 wt %. It is preferable to use zeolites with medium and large pores. Size of zeolites pores should range from 4 .ANG. in diameter and higher. The most preferable zeolites are with large pores (&gt;6 .ANG.) such as Y and mordenite.
Phenol derived from distillation of heavies and lights and still containing the impurities hydroxyacetone, mesityl oxide, alpha-methylstyrene, 2-methylbenzofurane and other carbonyl compounds but not containing water (water content less than 1 wt %, preferably less than 0.1 wt %) contacts the zeolite catalyst at a temperature of between about 120.degree. to 250.degree. C. (preferably 180.degree.-200.degree. C.) and space velocities 0.1-3.5 hr.sup.-1 (preferably 0.8-1.2 hr.sup.-1).
It has been discovered that said purification results in not less than 90% conversion of such impurities as mesityl oxide, hydroxyacetone, alpha-methylstyrene and other carbonyl compounds. It is important that the above mentioned impurities are converted into the products of condensation under the effect of the catalyst and purification conditions, so that these impurities can be separated from phenol by further rectification and removed from the process along with other "heavy" products, so-called "phenol tar". At high space velocities, (more than 0.8 hr.sup.-1) there is a rise in the level of 2-methylbenzofurane to an amount which can be easily removed from phenol by distillation. At low space velocities (less than 0.8 hr.sup.-1) and high temperatures (more than 200.degree. C.) there is a decrease in 2-methylbenzofurane amount (conversion is not more than 50%). Acidity of zeolites active centers can be stabilized by rare-earth elements (e.g. lanthanum and others). This results in a stable catalyst run over an extended time even at high micro-impurities content in the phenol to be purified. The useful life of the above mentioned catalysts is not less than three years.
The present invention is illustrated by the following examples which do not limit the invention and its claims but illustrate the application of a number of zeolite catalysts of the type suitable for phenol purification: